Groundwood bleaching process



Patented Oct. 10, 1944 GROUNDWOOD BLEACHING PROCESS Paul S. Brallicr and Edmund S. Nowicke, Niagara Falls, N. Y., asslgnors, by mesne assignments, to Staufler Chemical poration 01' California Company, a cor- No Drawing. Application December 2, 1940,

Serial N0. 368,226

3 Claims.

This invention relates to treatment of wood pulp, and has to do with a process for bleaching groundwood to render it suitable for use in paper making and analogous purposes.

Groundwood has the advantage that it gives a higher yield of pulp to a given quantity of pulp wood, but it has the disadvantage of possessing a color not Well suited for paper making. It is known to combine bleached sulfite pulp with groundwood to produce papers of better grades than newsprint. The added sulfite pulp contributes somewhat to the strength of the finished paper, but its principal function is to improve the color. Use of groundwood in making paper possesses certain desirable advantages, such as bulk, opacity and ink receptivity, in addition to its comparatively low cost. But the objectionable color of groundwood is a limiting factor and limits the ratio of groundwood pulp to sulfite pulp, in the known practice referred to.

So far as we are aware, groundwood pulp is ordinarily considered incapable of being bleached without serious loss in weight. That loss would destroy to a considerable extent, if not completely, the economic advantage of using groundwood. On the other hand, if that loss could be avoided and groundwood pulp could be bleached so as to remove its objectionable color without loss in weight, considerable advantages would result in that groundwood pulp would become practically available for use in greatly increased quantities in paper making. Our invention is directed to the attainment of that result. More specifically, our invention is directed to a process of bleaching groundwood pulp effective to remove the objectionable color thereof and to impart thereto a permanent increase in brightness,

while avoiding loss in weight of the pulp treated.

We have found that the desired results may be attained by treating groundwood pulp with a hypochlorite of an alkali or an alkaline earth metal, caustic soda and sodium silicate. The hypochlorite acts as a bleaching agent, the caustic soda serves to adjust the hydrogen ion concentration, or pH, to a proper value, and the sodium silicate acts as a bufier inmaintaining the pH at the desired level during the bleaching action. We have found that it is important that the pH value be maintained between 9.5

and 11, preferably 10.5.

The quantities of the various chemicals used will vary with the characteristics of the pulp being thereof; and adding the proper amount of hypochlorite and the caustic soda and sodium silicate in accordance with the density of the pulp to give the desired pH value of from about 9.5 to 11. The hypochlorite used is one of the group of alkaline earth metals, including barium, strontium and calcium; or one of the alkali metal groups including sodium and potassium.

By way of example, we give the following with respect to treatment of groundwood pulp in accordance with our invention:

. 200 grams of groundwood pulp, having a density of 2.5%, was heated to a temperature of 50 C. To this pulp were added 5 milliliters of a. so-

ing treated, and may best be determined by first ascertaining, by .known methods, the chlorine demand of that pulp to obtain the desired bleachlution of sodium meta silicate, containing gm./l. NazSiOa; l milliliter of sodium hydroxide solution, containing gm./l. NaOH; and 7 milliliters of a calcium hypochlorite solution, prepared by absorbing chlorine in limewater and havinga concentration of 100 gm/l. available (hlorine. The pulp and the added chemicals were thoroughly mixed together by stirring, and the mixture was maintained at a temperature of 50 C. for one hour and subjected to occasional vigorous agitation.

At the end of the hour, the pulp mixture was poured onto a suitable filter and the treating sol tion was filtered out, after which the pulp was washed several times with small volumes of water, which was filtered out. The pulp was then in condition for drying or for further processing, as desired.

Treatment of the pulp in the above manner resulted in a decided improvement in its color and produced an increase in brightness of from 5 to 6 points on the scale of a standard brightness meter. This increase in brightness is permanent, so far as we have been able to determine. The improvement in color is principally due, we believe, to the oxidizing efiect of the hypochlorite used, though, when an alkaline earth hypochlorite is used, precipitation of alkaline earth silicate in the pulp fibres may contribute to the color improvement.

In addition to improving the color of the pulp treated, an important feature of our process is that substantially none of the constituents of the groundwood pulp are extracted and there is no loss in weight. In fact there may be a slight increase in weight, since a portion of the chlorine in the hypochlorite seems to combine chemically with some constituent of the pulp, tending to increase its weight, and any alkaline earth silicate which may be precipitated in the pulp ilbe ill also tend to increase theweight or the Pulp.

The process 01' our invention. thus renders it ent inthewood for a period 01' approximately one'hour and at a temperature of approximately 50 C., to the action or'calcium hypochlorlte in the presence of sodium hydroxide in amount In practicing our process the chemicals used are in the approximate proportions of 60 milligrams of sodium meta silicate, 20 milligrams of sodium hydroxide, and 140'milligrams of available chlorine; per gram of air-dry pulp being treated. It will be understood, of course, that the proportions given are by way of example and may be varied to suit the characteristics of the groundwood being treated. Likewise, the temperature of the pulp mix during treatment may be varied and, within limits, the pH value, depending upon the condition and characteristics of the groundwood pulp undergoing treatment. While we have specifically named certain chemicals as suitable for use in our process, we do not wish to be limited to those particular chemicals since our invention comprehends the use of others suitable for the purpose. For example, sodium borate, in the form of borax or sodium meta borate, or potassium silicate, may be used as a buiier instead of sodium meta silicate, though the latter costsfless than either sodium borateor potassium silicate and for that reason is preferred. Also, potassium hydroxide may be substituted for sodium hydroxide for ad-.

justing the pH, but sodium hydroxide costs less and is preferred for that reason.

to adjust the pH of the mix. to a value of from about 9.5 to 11 and of sodium meta silicatein amount to maintain said pH value during the bleaching operation, thereby bleaching the'puip by the actionot the calcium hypochlorite and causing precipitation in the pulp of inorganic matter by interaction of the chemicals used, and filtering and washing the treated pulp.

2. The process of bleaching groundwood which comprises subjecting chemically untreated groundwood pulp containing substantially. all of earth hypochlorite in the presence of hydroxide in amount to adjust the pH of the chlorite and causing precipitation in'the pulp or inorganic matter by interaction of the chemicals used, and filtering and washing the treated pulp.

3. The process of bleaching groundwood which comprises subjecting chemicallyuntreated groundwood pulp containing substantially all of the lignin and other constituents originally pres-.

ent in the wood to the action of calcium hypochlorite in the presence of sodium hydroxide in The process is not limited to groundwood as pulp, as used herein and in the appended claims,

is to be construed as inclusive of groundwood pulp as such and of analogous materials such as those above referred to.

'such but applies equally to materials such as We claim: l. The process of bleaching groundwood which comprises subjecting chemically untreated groundwood pulp containing substantially all of the lignin and other constituents originally presamount to adjust the pH of the mix to a value of from about 9.5 to 11 and of sodium meta silicate in amount to maintain said pH value during the bleaching operation, with the calciumhypochloriteand-the sodium hydroxide and 'the sodium meta silicate present in approximately the milligrams, thereby bleaching the pulp by'thel action of the hypcchlorite and causing precipitation in the pulp of inorganic matter by interaction of the chemicals used, and filtering washing the treated pulp.

PAUL s. BRALLIER. EDMUND s. NowIcx'E 

